During drilling of the borehole drilling fluid is generally pumped through the drill string to the lower end of the string, from where the drilling fluid returns to surface via the annulus formed between the drill string and the borehole wall. The circulating drilling fluid transports the drill cuttings to surface, controls the wellbore pressure, and cools the drill bit.
A frequently encountered problem in the practice of drilling wellbores is leakage of drilling fluid from the borehole into the surrounding earth formation. Some leakage of fluid is generally considered allowable, however in many instances the amount of leakage is such that further drilling is not allowable without first taking corrective measures. Such heavy fluid losses can occur, for example, during drilling through depleted sandstone reservoirs and/or through unstable shales. It has been tried to stabilise the shales by applying a drilling fluid having a relatively high specific weight. However the weight of such heavy drilling fluid can be close to, or in excess of, the fracturing pressure of neighbouring sandstone formations. Conventional corrective measures include pumping of Lost Circulation Material (LCM) through the wellbore in order to plug the formation, pumping cement into the wellbore, or installing a casing or liner in the wellbore at the location of the fluid losses. The latter is the only feasible option in case the fluid losses are severe. Until now this has been done by retrieving the drill string and running the casing/liner into the borehole, which is a time consuming and costly procedure. Moreover, temporary measures to reduce the losses to acceptable levels have to be taken before retrieving the drill string from the borehole.